Aircraft: B757
Phase of flight: Parked
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy

[Flight] crew called me up during their prefight to show me the First Officer had a message flag in his lower right display that said DCU; and [DCU] was in a White box. Neither I nor the crew was aware what it was or meant and I checked the EICAS Maintenance Page and there was no message there or any status message and the First Officer's displays were normal. I called Maintenance Control and discussed it with the Representative (Rep) X and he wasn't real sure what it was either; but we decided that we could get relief using MEL Data Concentrator Units-Flat Panel Display Data Concentrator Unit (DCU) and that since it was on the First Officer's side we would use the [Right] 'R' DCU. As that is the normal (norm) in the layout of the B757. We accomplished the Maintenance portion of [MEL] verifying that the Captain's and First Officer's DCU's could be isolated using the Electronic Flight Instrument (EFI) switch. The Crew; Maintenance Control Rep; and I were in agreement that this was the appropriate course of action and I dispatched the B757 aircraft 19 minutes late. I read the Corrective Action accomplished in ZZZ1 and saw where the Aircraft Maintenance Technician (AMT) there said that the wrong DCU had been MEL'd and that he would 'Close' this out and write-up the correct DCU. He performed the EFIS Adjustment (Adj)/Test and signed it off without writing the correct one up. And I still didn't know which one it should have been until I talked to Maintenance Control Representative Y; who knew much more and was able to explain to me how it all worked. It all goes back to 'that' white outlined box that had DCU in it. The white box is for the Center DCU and saying [indicating] that the DCU was bad; or has had a fault entered; and the amber box would be for the 'Right' DCU or First Officer's side. Apparently there was no real fault as it passed the Adj/Test and I assumed it was the Center DCU that was tested. But the sign-off doesn't say. So now I know and I hope the word gets out about this because I don't recall this being addressed in our two week systems class. This is the first time I've had anything like this since working this aircraft. As I mentioned; this was the first time I've had a write-up like this and it was unfamiliarity with how the system works and what the white and amber colors meant. Also nothing against the Maintenance Control Reps; but the level of knowledge differs depending on who you talk to. Maintenance Controller Y was very helpful in explaining the system to me...unfortunately after the fact. In my case; I know now what the amber and white boxes mean. Maybe training can review these kinds of events and incorporate these lessons learned into their training. A Line Aircraft Maintenance Technician (AMT) describes his efforts to determine the meaning of a message flag with 'DCU' inside a white box on the First Officer's (F/O) lower right Display. The B757-200 aircraft's Center Data Concentrator Unit (DCU) had faulted; but was incorrectly deferred as the 'Right' DCU with the concurrence of the flight crew; Maintenance Controller; and Technician.